The term fuel metering unit (FMU) denotes a unitary component which receives fuel from a fuel supply and provides a controlled output of fuel to an engine in accordance with one or more control signals applied to the FMU by the Electronic Engine Controller (EEC) or, as it is frequently known in aircraft applications the Full Authority Digital Engine Controller (FADEC). In its simplest concept the FMU is a single fuel metering valve incorporating a motor or the like for driving the valve between open and closed positions and a position transducer which supplies to the EEC signals representative of the operative position of the valve. However, conventionally the FMU will include other control components in association with the metering valve such as, for example, a Pressure Drop Regulator and Spill Valve (PDSV) which controls the pressure drop across the metering orifice of the metering valve, and, a Pressure Raising and Shut Off Valve (PRSOV) which ensures a minimum system fuel pressure and can disconnect the engine from the metering valve when appropriate. In certain applications the FMU as described above may be an integral part of the fuel pumping arrangement of the fuel supply system.
In a conventional aircraft gas turbine engine fuel supply system the EEC is programmed with a predetermined fuel control law to which the FMU adheres. Thus the fuel control law will contain a predetermined table of FMU input signals to be supplied to the metering valve to vary the opening of the metering valve, so as to achieve predetermined flow values of the fuel supplied from the FMU to the engine. In order that the FMU can adhere to the EEC stored fuel control law the FMU is initially subjected to a detailed and rigorous calibration procedure during and after assembly, and before the FMU enters service. The calibration procedure is used to adjust the metering valve, and other components of the FMU when present, so that for a given fuel flow demand signal from the EEC a predetermined output of fuel from the FMU is achieved.
The calibration procedure for the FMU is time-consuming and therefore expensive. Typically calibration involves making adjustments, accurately shimming position sensors and springs, and may involve repeated assembly and disassembly of the FMU. Moreover, every FMU which is manufactured to suit a particular application must go through the same calibration process so that after calibration its performance characteristics are as close as possible to the performance characteristics of every other calibrated FMU for use in that application, and to the fuel control law stored within the EEC. In order to simplify the calibration of a large number of FMUs to be within close limits it is desirable to utilise, in the FMUs, metering valve position sensors which are all produced to a similar high level of accuracy. The same is also true of the devices which drive the metering element of the metering valve to set the opening of the valve and also the other valves associated with the metering process. Such accurately produced components are inherently expensive.
The aforementioned problems are exacerbated in so-called “dual-channel” control systems. In such a control system it is recognised that the mechanical components of the FMU are relatively robust, but that the electrical, and electromechanical components, notably the position transducer of the metering valve, are more susceptible to damage or failure, and in a dual channel arrangement a single metering valve will have two position transducers operating in parallel. Clearly both transducers should be as near identical as possible so that the performance of the FMU in relation to the signal supplied by the EEC is the same irrespective of which channel is operative. Clearly therefore the dual channel approach adds to both the equipment cost, and the time taken for calibration.
A further disadvantage of a conventional fuel control arrangement is that it is costly to make in-service changes. If a change is made to a conventional FMU design after the unit has entered service and this change affects the basic flow versus metering valve position calibration, it is very expensive to change the fuel control law software in all the in-service EECs. Also there would be the potential to miss-match EECs having the amended control law with pre-modification standard FMUs. It is therefore usual to implement such design changes by modifying the FMU metering valve metering profile, so that modified FMUs will obey the unchanged fuel control law, and this can often be difficult and expensive to achieve.
Irrespective of the calibration disadvantages described above there are some maintenance disadvantages associated with conventional FMU's. In particular, when an FMU is removed from an engine fuel supply system for maintenance purposes it is necessary to associate with the FMU a hard copy print from the associated EEC of any fault identifier codes which have been stored by the EEC in relation to that FMU during its use. Alternatively a manually created service log will need to be consulted so that the maintenance operative can be made aware of service life data of the FMU.
It is a primary object of the present invention to provide an FMU, and engine control system utilising an FMU, in which the aforementioned disadvantages are minimised or obviated.